A light-emitting element using a light-emitting material has advantages of thinness, lightness in weight, and so on, and is expected to be applied to a next-generation display. Further, since the light-emitting element is a self-light-emitting type, the light-emitting element is superior to a liquid crystal display (LCD) in high visibility without a problem such as a viewing angle.
A basic structure of a light-emitting element is a structure having a light-emitting layer between a pair of electrodes. Voltage is applied to such a light-emitting element, so that holes injected from an anode and electrons injected from a cathode are recombined in a light-emission center of the light-emitting layer to excite molecules; thus, light is emitted by emitting energy when the excited molecules returns to the ground state. Note that the excited state generated by the recombination includes a singlet-excited state and a triplet-excited state. The light emission can be obtained in either of the excited states. In particular, the light emission when the singlet-excited state returns to the ground state is referred to as fluorescence, and the light emission when the triplet-excited state returns to the ground state is referred to as phosphorescence.
By the way, in the case where a light-emitting element is incorporated as a display portion of a device using electromotive force from a battery, for example, an electronic device such as a mobile phone, a camera, or a portable music reproducing device, low power consumption is required in order to continually use the device including the display portion with the light-emitting element for a long time. Development of an efficient light-emitting element is required in order to achieve low power consumption; therefore, a light-emitting material having excellent light-emitting efficiency has been sought for, and many studies thereof have been done (Reference 1: International Publication No. 2000/040586 pamphlet).